1. Field of the Invention
This invention relates to apparatus for detecting a sudden rise in pressure of a source thereof. In particular, it relates to apparatus capable of detecting a sudden incipient rise in pressure in a containment system which could result in a destructive overpressure in that system.
Low strength containment and process vessels used in dust collectors, process equipment, dryers, ovens, mills, bucket elevators, storage bins, hoppers and similar devices are subject to catastrophic destruction if an explosion occurs in the vessel or adjacent conveying ducts or pipes, or associated processing units.
Because of normal pressure fluctuations that occur within the containment vessel, it has heretofore been difficult to effectively distinguish between the incipient rise in pressure within the containment system or associated units which signals the onset of an explosion, thus necessitating activation of explosion suppression or isolation equipment, as compared with the pressure fluctuations that may occur during normal operation of the system.
2. Description of the Prior Art
A number of different industrial processes have containment systems which receive material for collection for further processing. For example, hammermills, solid material grinders, and similar comminuters for products used in the agricultural, horticultural, food or chemical fields are provided with collectors or filter devices for preventing small particles from being discharged into the surrounding atmosphere. Bag houses containing cloth filters have long been used to remove particulate materials from product conveying air before it is discharged into the air. These bag houses have wall structure which protects the surrounding atmosphere but the containment area is of low strength.
Similarly, cyclones for separating larger particles from air streams are of relatively thin wall construction because the normal pressure contained therein is not above atmospheric to a significant degree.
Other collection and containment systems have similar low strength surrounding or containing wall structures. They are designed for the functional purpose of maintaining an air stream under control while particulate material is either removed from the air, or conveyed to another processing area in the air stream.
In instances where a containment system of the type described is connected to downstream processing units, there is always a danger of an explosion which can cause catastrophic over pressures which at the very least destroy the containment vessel or facility. In certain instances, disastrous fires may occur, surrounding peripheral equipment may be subjected to dangerous shock waves, and adjacent workers may even be injured.
Explosion protection systems have previously been provided for protecting containment vessels against catastrophic explosions, but their ability to sense an explosion early in its inception has been somewhat limited because of the need to set the over pressure sensing limit at a level which prevents false alarms or suppression activations as a result of normal pressure fluctuations which occur in the containment system.
The present invention utilizes a dual diaphragm, differential pressure switch of the type used for many years in various applications where the pressure in one part of a processing system may vary relative to the pressure in another part of the system.
Differential pressure sensors per se are not new. Shaffer in U.S. Pat. No. 801,922 discloses a dual diaphragm differential pressure switch as a part of an automatic fire sprinkler system.
In Shaffer, a rod extends between opposed diaphragms and supports a finger which when moved by deflection of the diaphragms operates a control switch. Actuation of the switch initiates an alarm at a central station whenever the equilibrium in the distribution piping of the system is disturbed, either in response to a higher than normal water pressure, or decrease in pressure below a fixed level by virtue of a leak in the piping. However, Shaffer's dual diaphragm pressure switch is not insensitive to normal pressure fluctuations in the sprinkler system.
Compton in U.S. Pat. No. 3,398,991 discloses an automatic vehicle control system which is described as preventing overturning of the vehicle on sharp curves. The centrifugal force detecting, anti-overturn system is sensitive to centrifugal force and is operable to apply braking force to the individual wheels when a vehicle is in a precarious leaning position.
Somes, et al. in U.S. Pat. No. 2,359,168 describe a flow responsive device designed to control operation of an electric switch. A movable member responsive to pressure variations in the flow of fluid through an associated conduit shift the actuating member of an electric switch in accordance with flow conditions within the conduit. The patentees' unit is said to be especially useful for processes involving heat treating of metals by electromagnetic heating and subsequent quenching, where the heating and cooling must be accurately coordinated and controlled. To do so, the flow of hot and cold fluids are sensed and a proper balance maintained therebetween.
U.S. Pat. No. 2,645,248 to Baker discloses a differential pressure device capable of responding to an overpressure condition. The Baker patent seeks to minimize the spacing between opposed diaphragms to decrease the size of the unit and the number of parts required to determine the pressure differential between different sources of pressure.
Rice in U.S. Pat. No. 4,081,636 describes a differential pressure cut-out switch for monitoring the lubrication oil pressure in a sealed refrigeration system compressor. The switch has a low pressure operating point assembly connected to the crank case and a high pressure operating point assembly communicating with the oil pump. A contactor switch unit in the nature of a sensor controller is movable to a first position to provide an open circuit and a second position connecting a resistance of a finite value in the control circuit as a function of the difference between the two system pressures. The differential pressure cutout switch when employed with the sensor controller enables distinguishment between adequate, marginal and inadequate lube oil pressure.
Canadian Pat. No. 636,289 describes an electro-pneumatic differential pressure switch system for automatically operating electrical switches with high switch sensitivity. The system is designed to effect operation of the switches over a wide range of differentially opposed pressures, but as a result of a very small pressure difference. Canadian Pat. No. 697,112 describes a pressure control switch for aircraft wherein the switch is actuated when there is a predetermined relationship between dynamic pressure measured by a pitot minus the static pressure. An aircraft mach number speed therefore may be expressed as a function of the ratio of the differential pressure and the static pressure. In order to obtain the required relationship, diaphragms are used to define three pressure containing areas.